From U.S. Pat. No. 4,371,413 (see also German patent publication DE-OS No. 30 26 038), an apparatus is known for applying transverse weld seams to a thermoplastic foil which includes a conveyor which is synchronized and intermittently advanced with respect to a welding unit.
Controlling the movement of the conveyor and of the welding unit is a control unit which includes a drive disk supported on a crankshaft and provided with an eccentric crank pin which is displaceable in a slot of a rocker to oscillate the latter.
Mounted on the rocker is an adjusting spindle which supports a movable slide block. Connected to the slide block is one end of a coupling rod, whose other end supports a gear segment which is operatively aligned with the conveyor.
By adjusting the slide block along the spindle, the stroke of the oscillations of the rocker about its respective journal or axis are controlled and transmitted to the gear segment which accordingly advances the conveyor via a suitable transmission
In order to provide an intermittent advance of the conveyor, a clutch-brake unit connected between the conveyor and the transmission uncouples the gear segment from the conveyor and brakes the latter until being at a standstill or connects the respective parts to provide the advance of the conveyor.
In this prior art apparatus, the rocker oscillates about a stationary axis and is connected to the drive disk which is designed as a gear and provided with a non-adjustable eccentric pin. Since the coupling rod is not directly connected to the eccentric pin but rather to the slide block of the rocker, the eccentric pin is only used to convert the rotation of the disk into the oscillating motion of the rocker while the actual kinematics are determined by the adjustable slide block. Consequently, the conveyor is brought to a standstill over a predetermined period during each cycle for thermal welding while the duration of the standstill period is controlled by the adjustable spindle.
The disadvantage of this system is that a precise or fine adjustment of the conveyor is not possible, i.e. there is no provision to correct the position of the weld seam in dependence on the print register. Frequently the print register is altered because of inherent tolerances or due to stretching with respect to the plastic foil web so that the respective weld seams are not accurately associated with each print image. Moreover, this apparatus does not provide for a coarse setting enabling adjustment to different kinds or sizes of print registers.